What causes transgender traits?

As you read this, many people are devoting their lives to determining the causes of gender diversity. These people have many ideological reasons for undertaking this effort, usually to prove some belief they hold about how the world is structured. Some of these motivations are noble; others are not so noble.

For this reason, the stakes are high. For instance, a medical cause could allow for early detection and treatment, and might help alleviate the stereotype that this is a “lifestyle choice.” Alternately, that same information could be used to screen fetuses and terminate pregnancies.

Laws may even be affected if experts determine causes (sometimes called etiology). In the past, important laws have been passed based on medical findings (or the lack thereof), and trans people have won and lost legal cases based on what evidence about gender identity and expression was presented in court.

Current controversies

1. Transsexualism as a “disorder”

Please note that it is considered controversial to think of our condition as a gender identity “disorder,” as described in this document. See Depathologizing gender identity by Katherine Wilson, Ph.D. for more on this.

I have also written an essay examining disease models of gender identity.

2. Concern about a “cause”

Noted scholars such as Daryl Bem have argued that asking “What causes homosexuality?” is “both politically suspect and scientifically misconceived.” The same argument can be made regarding gender identity issues. See Exotic Becomes Erotic (PDF file: requires reader) for an interesting overview of this concept applied to sexual orientation.

3. Congenital brain development issues

A congenital trait is something that is present at birth. Several scientific hypotheses examine the effects of hormones during fetal development. One notable hypothesis examines a synthetic hormone called diethylstilbestrol (DES) and its effect on developing fetuses. This drug was administered between 1938 and 1971 to prevent miscarriages, but it has since been linked to several developmental issues in exposed fetuses. Some transwomen born during that period have created essays and discussion groups.

The overview below summarizes recent scientific research. While the studies listed below do not provide conclusive evidence, they suggest that this hypothesis has enough scientific credibility to merit additional research. The text below is taken from the document housed on the GIRES site.

Terry Reed of GIRES sent an update in February 2006, stating that a more comprehensive document is available via their website:

Atypical Gender Development – A Review

(full next in PDF)


That document supersedes the text below.


Definition and Synopsis of the Etiology of Adult Gender Identity Disorder and Transsexualism

1. Gender Identity Disorder is defined as an incongruence between self identification as male or female and the physical phenotype. The experience of this incongruence is termed Gender Dysphoria. The most extreme form, in which individuals need to adapt their phenotype with hormones and surgery to make it congruent with their gender identity, is called transsexualism. Those individuals experiencing this condition are referred to as trans people, that is, trans men (female-to-male) and trans women (male-to-female).

2. Transsexualism can be considered to be a neuro-developmental condition. Several sexually dimorphic nuclei have been found in the hypothalamic area of the brain (Swaab & Fliers, 1985; Allen & Gorski, 1990; Swaab et al, 2001). Of particular interest is the sexually dimorphic limbic nucleus called the central subdivision of the bed nucleus of the stria terminalis (BSTc) which appears to become fully mature in the human brain by early adulthood. In males the volume of this nucleus is almost twice as large as in females and its number of neurons is almost double (Zhou et al, 1995; Kruijver et al 2000; Chung et al 2002).

3. In the case of transsexualism this nucleus has a sex-reversed structure. This means that in the case of trans women (n=7), the size of this nucleus and its neuron count is in the same range as that of women in the general population. In the only available brain of a trans man, the structure of this nucleus was found to be in the range of males in the general population. It is hypothesised that this male-like BSTc will be present in other trans men as well. These findings were independent of sexual orientation and of the use of exogenous sex hormones. It is inferred that the sexually dimorphic BSTc is an important part of a neural circuit involved in the development and establishment of gender identity (Kruijver et al, 2000).

4. Sexual differentiation of the mammalian brain starts during fetal development and continues after birth (Kawata, 1995; Swaab et al, 2001). It is hypothesised that in humans, in common with all other mammals studied, hormones significantly influence this dimorphic development although, at present, the exact mechanism is incompletely understood. It is also postulated that these hormonal effects occur at several critical periods of development of the sexual differentiation of the brain during which gender identity is established, initially during the fetal period, then around the time of birth; and also post-natally. Factors which may contribute to an altered hormone environment in the brain at the critical moments in its early development might include genetic influences (Landen, 1999) and/or medication, environmental influences (Whitten et al., 2002), stress or trauma to the mother during pregnancy. (Ward et al., 2002; Swaab et al., 2002)

5. Gender identity usually continues along lines which are consistent with the individual’s phenotype, although there are a very small number of children who experience their gender identity as being incongruent with their phenotype. However, adult outcomes in such cases are varied and cannot be predicted with certainty. It is only in a minority of these children that, regardless of phenotypical socialisation and nurture, this incongruence will persist into adulthood and manifest as transsexualism. (di Cegli, 2000; Prosse, 1998; Ekins, 1997; Bates, 2002; Ekins & King, 2001; Green, 1987)

6. As stated, in trans people, a sex-reversed BSTc has been found. This specific sex-reversed brain organisation in trans people provides persuasive evidence of a biological predisposition for transsexualism. This evidence for an innate biological etiology is reinforced by other studies, one example of which, indicates a higher than average correlation with left-handedness (Green & Young, 2001). Where the predisposition for transsexualism exists, psycho-social and other factors may subsequently play a role in the outcome, however, there is no evidence that nurturing and socialisation in contradiction to the phenotype can cause transsexualism, nor that nurture which is entirely consistent with the phenotype can prevent it (Kipnis &Diamond, 1998). There is further clear evidence from the histories of conditions involving anomalies of genitalia, that gender identity may resolve independently of genital appearance, even when that appearance and the assigned identity are enhanced by medical and social interventions (Reiner, 2002; Kipnis & Diamond, 1998; Diamond and Sigmundson, 1997). It is not possible to identify one single cause for transsexualism: rather, its causality is highly complex and multifactorial. The condition requires a careful diagnostic process, based largely on self-assessment, facilitated by a specialist professional.

7. In conclusion, transsexualism is stongly associated with the neurodevelopment of the brain. (Zhou et. al., 1995; Kruijver et. al., 2000). The condition has not been found to be overcome by contrary socialisation, nor by psychological or psychiatric treatments alone (Green, 1999). Individuals may benefit from an approach that includes a programme of hormones and corrective surgery to achieve realignment of the phenotype with the gender identity, accompanied by well-integrated psychosocial interventions to support the individual and to assist in the adaptation to the appropriate social role (Green and Fleming, 2000). Treatments may vary, and should be commensurate with each individual’s particular needs and circumstances.

[1] The term ‘gender identity’ is used, in the UK, to indicate the self-identification as male or female. However, terminology varies around the world, and the term ‘sexual identity’ is preferred by many in the US. (pace Professor Milton Diamond). See “Sex and Gender are different: Sexual Identity & Gender Identity are Different”, (2000) Clinical Psychology & Psychiatry, Vol 7 (3):320-334.

[2] The transsexual condition is also referred to in various ways (Diamond M, 2002 In Press) “What’s In a Name? Some terms used in the discussion of Sex and Gender”. Transgender Tapestry.

n.b.The UK government recognises that transsexualism is not a mental illness. See Lord Chancellor’s Department – government policy concerning transsexual people. http://www.lcd.gov.uk/constitution/transsex/policy.htm

Signatories {original authors are asterisked}

Dr Henk Asscheman, MD, PhD (The Netherlands)*
Professor Mike Besser, DSC, MD, FRCP, SmedSci. (UK)
Dr Susan Carr, MPhil. MFFFP. DDRCOG. (UK)
Dr Domenico di Cegli, FRCPsych., DIP. PSICHIAT (Italy) (Child Section) (UK)*
Professor Peggy Cohen-Kettenis PhD (The Netherlands)
Professor Mickey Diamond , PhD (Chair) (USA)* 
Professor Louis Gooren, MD, PhD (The Netherlands)
Dr Frank Kruijver, MD (The Netherlands)*
Dr Joyce Martin, MRCGP, MB ChB, D.Obst.RCOG. (UK)*
Dr Z-J Playdon, BA(Hons), PGCE, MA, MEd, Phd, DBA, FRSA. (UK)*
Mr David Ralph, MBBS, BSc, FRCS, MS. (UK)
Mrs Terry Reed, JP, BA(Hons),. MCSP, SRP, Grad Dip Phys.(UK)*
Dr Russell Reid, MB. ChB, FRCPsych. (UK)*
Professor William Reiner, MD. (USA)
Professor Dick Swaab , MD, PhD. (The Netherlands)
Mr Timothy Terry, BSc, MB, BS, LRCP, FRCS (Urol), MS (UK)
Mr Philip Thomas MBBS, FRCS (Urol). (UK)
Dr Philip Wilson, DPhil MRCP MRCPCH FRCGP
Dr Kevan Wylie MB, MmedSc, MD, MRCPsych, DSM. (UK)


1. Allen LS & Gorski RA (1990) Sex Difference in the bed nucleus of the stria terminalis of the human brain, J Comparative Neurology 302, 697-706.

2. Bates DJ, (2002), Locating the transsexual narrative in the gendered landscape. The University of Waikato; New Zealand. 437 pages

3. Chung WCJ, De Vries GJ, Swaab D, (2002), Sex differentiation of the bed nucleus of the stria terminalis in Humans may extend intp adulthood, J of Neuroscience , 22(3) 1027-1033.

4. Diamond M & Sigmundson HK, (1997), Sex reassignment at birth. Long term review and clinical implications. Archives of Pediatrics and Adolescent Medicine, 151, 298-304

5. Di Cegli D, (2000) Gender identity disorder in young people, Advances in Psychiatric Treatment, vol 6, 458-466.

6. Ekins R, (1997); Male Femaling. London, New York, Routledge, 185 pages

7. Ekins R & King D, (2001) Telling body transgender stories in Unseen Genders: Beyond the Binaries, editors: F Haynes & T McKenna. Peter Lang, New York

8. Green R & Fleming DT, (2000); Transsexual Surgery Follow-up: Status in the 1990s, Annual Review of Sex Research, editor J Bancroft, vol 1 163-174.

9. Green, R (1987) The Sissy Boy Syndrome and the Development of Homosexuality, New Haven CT, Yale Univ.

10 Green, R (1999) Cited in Bellinger v Bellinger, Ct of Appeal, para 32 July 17th ( Judgement,2001)TLR 22-11-2000

11. Green R, & Young R, (2001)Transsexualism, Left-handedness may share origins, Archives of Sexual Behaviour, 30:565-574.

12. Kawata M, (1995), Roles of steroid hormones and their receptors in structural organization in the nervous system. Neroscience Res, 24, 1-46.

13. .Kipnis K and Diamond M, (1998) Pediatric ethics and the surgical assignment of sex, J Clinical Ethics, 9(4) 398-410

14. Kruijver FPM, Zhou J-N, Pool CW, Hofman MA, Gooren LJG, Swaab DF. (2000) Male to Female Transsexuals Have Female Neuron Numbers in a Limbic Nucleus, J Clinical Endocrinology and Metabolism, Vol 85, No 5 2034-2041

15. Landen M (1999). Transsexualism, Epidemiology, phenomenology, aetiology, regret after surgery, and public attitudes. In Press. Institute of Clinical Neuroscience, Goteborg University, Sweden.

16. Prosser J, 1998; Second skins: The body narratives of transsexuality. New York, Columbia University Press. 270 pages.

17. Swaab DF & Fliers E. (1985) A sexually dimorphic nucleus in the human brain, Science, 228, 1112-1115.

18. Swaab DF, Chung WCJ, Kruijver, FPM, Hofman MA, Ishunina TA, (2001), Structural and functional differences in the human hypothalamus, Hormones and Behavior, 40, 93-98

19. Swaab DF, Chung WCJ, Kruijver FPM, Hofman MA. Hestiantoro A. (2002). Sex differences in the human hypothalamus in the different stages of human life. Neurobiology of aging, in press.

20. Reiner WG, (2002); Sexual identity and cloacal exstrophy. (2002) In press.

21. Ward OB, Ward IL, Denning JH, French JA, Hendricks SE. (2002) Postparturitional testosterone surge in male offspring of rats stressed and/or fed ethanol during late pregnancy. Hormones and Behavior, 41:229-235

22. Whitten PL, Patisaul HB, Young LJ, (2002), Neurobehavioral actions of coumestrol and related isoflavonoids in rodents, Neurotoxicology and Teratology 24: 47-54, 2002

23. Zhou J-N, Hofman MA, Gooren LJG, Swaab DF (1995b); A sex difference in the human brain and its relation to transsexuality, Nature, 378, 68-70.

Funded by Gender Identity Research & Education Society, the King’s Fund & the BCC Trans Group. The King’s Fund bears no responsibility for the text.

Reproduced here with the kind written permission of Dr. Diamond.

See a longer version of this document at: Atypical Gender Development – A Review

Further reading

LINK: A defining moment in our history: examining disease models of gender identity

LINK: Atypical Gender Development – A Review by GIRES

LINK: Depathologizing gender identity by Katherine Wilson, Ph.D.

LINK: What Causes Transsexualism? by Professor Lynn Conway

LINK: Biological Correlations of Atypical Gender Identities

LINK: Exotic Becomes Erotic by Daryl Bem, Ph. D.